PROJECT SUMMARY/ABSTRACT Breast cancer is the most common non-skin cancer among females in the United States (US) and the second leading cause of cancer death in women. Although less than 10% of all breast cancers are diagnosed among women younger than age 45, incidence during the past 30 years has increased faster among younger women than among women older than 40. Although research has recently begun characterizing the economic burden of breast cancer among younger women, the vast majority of the literature on the economic impact of breast cancer has focused on early stage cancers among older women (aged 65 and older). More research is needed to understand the economic impact of metastatic breast cancer among women at all stages of life. Medical treatment for metastatic breast cancer is not intended to ?cure? the cancer but rather to extend the life of the patient and/or improve the patient's quality of life. Several new treatments for metastatic breast cancer have been introduced in recent years. However, more studies are needed that estimate the value of oncology clinical pathways for metastatic breast cancer, as these treatments can be costly with limited survival benefit. The overall goal of this study is to assess the economic burden of metastatic breast cancer across the lifetime and to evaluate the cost effectiveness of alternative clinical pathways for metastatic breast cancer. Our study will estimate the costs of metastatic breast cancer to women in three age groups: younger (aged 18 to 44), midlife (aged 45 to 64) and older (aged 65 and older). We will estimate three types of costs: medical costs, the cost of not being able to work, and the cost of lost earnings from dying prematurely (Aim 1). We will then scale the estimates of the per-person costs to the population level and use US Census projections to project the overall economic burden of metastatic breast cancer out through the year 2030 (Aim 2). Finally, we will estimate the cost-effectiveness of different oncology clinical pathways in the first to third line treatment of patients with endocrine-sensitive, endocrine-refractory or triple negative, and HER2- overexpressing metastatic breast cancer among younger, midlife and older women. While several treatment options for metastatic breast cancer may increase a patient's life, the amount of extra time provided and the cost of the treatment to health insurance and the patient can vary dramatically. When comparing oncology clinical pathways, cost effectiveness analysis provides information about the extra cost per unit of additional healthy life-years gained from choosing one treatment over another. The economic cost data produced by this study will help decision makers allocate scarce public health resources more effectively for the prevention and treatment of metastatic breast cancer. The cost projections can highlight cost drivers for women with metastatic breast cancer and motivate appropriate prevention and early detection public health activities. This study will also identify cost-effective oncology clinical pathways to improve the value of care and health-related quality of life of women of all ages with metastatic breast cancer.